Recently, resistance exercises performed on an unstable surface have become part of athletic training and rehabilitation. Accordingly, their role in performance and health-oriented strength training has increasingly emerged as a matter of interest to researchers and conditioning specialists. A more pronounced activation of stabilizing muscles is assumed to be the main feature of instability resistance exercises. This assumption has been proven by EMG studies, which have highlighted significantly greater electromyographic activity of trunk-stabilizing muscles during exercises under unstable as compared to stable conditions. Intervention studies also demonstrated an enhanced improvement of trunk stability after training programs utilizing unstable devices as compared to floor exercises. Findings indicate that instability resistance training may facilitate the neural adaptation of trunk-stabilizing muscles, resulting in an improvement in trunk stability. However, both acute and long-term responses of primarily activated muscles to exercises performed on an unstable surface remain a matter of debate. It has been established that there is a significantly lower peak isometric force and rate of force development during resistance exercises under unstable as compared to stable conditions. In addition, the power output was compromised when exercises were performed on unstable surfaces. However, we have demonstrated that this effect depends on the type of exercise, instability device used, weight lifted, subject's training background, and so forth. Our findings on muscular power in the concentric phase of resistance exercises with different weights under stable and unstable conditions complement this review. Applications of instability resistance exercises for the improvement of neuromuscular functions in the physically active, plus for those following anterior cruciate ligament reconstructions, are also presented.
Keywords: BOSU ball; Chest press; Power output; Squat; Swiss ball.